Modulation can be defined as alteration of one or more characteristics of a certain waveform as a function of another signal that represents information. In radio frequency communication systems, a waveform the characteristics of which are altered is typically a sine wave. In this document a modulated sine wave is called a carrier and a signal that represent information is called a modulation signal. There are several methods for modulating a carrier, like linear amplitude modulation, frequency modulation, and phase modulation. In quadrature amplitude modulation (QAM) there are two sine waves being in a 90 degrees phase shift respect to each other. Each sine wave is modulated with amplitude modulation, and the modulated sine waves are summed together to form a final modulated signal. Quadrature amplitude modulation is called also IQ-modulation, illustrating a system having one ‘in-phase carrier’ and another 90 degrees phase shifted ‘quadrature carrier’. The QAM-modulation is however a complex modulation method requiring two parallel amplitude modulation branches in a modulator circuitry. This kind of complexity increases manufacturing costs, required space, and power consumption of a modulator circuitry. For example, in a mobile phone it is very important that signal processing elements, e.g. modulators, are small and cost effective and the power consumption of them is low. Therefore, there is a tendency towards frequency/phase modulation based on direct modulation of an output signal of a controllable oscillator that is a part of a phase locked loop. A controlled oscillator can be, for example, a voltage controlled oscillator (VCO) whose input voltage determines the output frequency.